In prior art, in Ziegler-Natta catalysts for olefin polymerization or copolymerization, an active component generally contains titanium, magnesium and halogen as main components, and when being used in polymerization, a promoter aluminum alkyl compound and an external electron donor component are added. For stereospecific polymerization of olefins, especially stereospecific polymerization of propylene, if no external electron donor is added, the isotacticity of the polymer resulted by using most of the catalysts is low, and generally lower than 90%, which has adverse effects on industrial production and application of the polymer. Therefore, for most of the catalysts for propylene polymerization, addition of the external electron donors plays an important role.
Currently, the types of external electron donor have been extended from original benzoate esters to presently extensively used organosiloxanes. U.S. Pat. No. 4,562,173 discloses a catalyst component for polymerization of olefins, which contains an organosiloxane compound represented by general formula SiRm(OR′)4-m, where R is hydrogen, alkyl or aryl, R′ is alkyl or aryl, and m is in the range of 0 to 4. The embodiments disclosed in the patent demonstrate that, by using phenyltriethoxysilane as the external electron donor for propylene polymerization, the isotacticity of the product polypropylene is significantly higher than that of the product obtained by using ethyl p-methyl benzoate as the external electron donor in comparative embodiment. U.S. Pat. No. 4,927,797 discloses an external electron donor represented by general formula R1(OR2)Si(OR3)R4 that used with a specified Toho-type titanium magnesium catalyst system in pair, where R1 is alkyl or cycloalkyl with large steric hindrance and containing at least one secondary carbon or tertiary carbon atom, R2 and R3 are alkyl or aryl, and R4 is alkyl carrying primary carbon atom and directly connected to Si atom. Preferably, R1 is cyclohexyl or tert-butyl; R2 and R3 are methyl, ethyl, propyl or butyl, and may be the same or different; R4 is a group containing 1 to 6 carbon atoms. More preferably, the compound is cyclohexylmethyldimethoxysilane. The embodiments disclosed in the patent demonstrate that, compared with diphenyldimethoxysilane, using cyclohexylmethyldimethoxysilane as the external electron donor of the specified catalyst has the advantages of smoothness in isotacticity adjustment and hydrogen control, and slow attenuation, thus facilitating stable operation of the industrial unit. Patent No. EP0350170 discloses a process for polymerizing olefins and a polymerization catalyst therefore. The catalyst contains an external electron donor represented by general formula SiR21R22m(OR23)3-m, where R21 is cyclopentyl, cyclopentenyl or cyclopentadienyl and groups derived therefrom, R22 and R23 each represented a hydrocarbon group and may be same or different, 0≦m≦3. The compound was preferably dicyclopentyldimethoxysilane. The embodiments in the patent demonstrate that, when being used in propylene homopolymerization, the catalyst has the advantages of high activity and high polymer isotacticity; and when being used in preparation of randomly copolymerized polypropylene film material, the catalyst has the advantages of high ethylene content, low initial heat sealing temperature, and high transparency. Patent No. CN1176258 discloses a catalyst system for propylene polymerization and copolymerization and a polymerization method. The catalyst system contains the Ziegler-Natta catalyst in combination with an external electron donor represented by general formula SiRm(OR′)4-m that are conventionally loaded on a carrier, where R is a group selected from alkyl, cycloalkyl, aryl and vinyl; R′ is alkyl; m is 0-3, when R is alkyl, R and R′ may be the same; when m is 0, 1 or 2, R′ may be the same or different; and when m is 1, 2 or 3, R may be same or different. Particularly, the external electron donor in the catalyst system may be selected from cyclohexylmethyldimethoxysilane, diisobutyldimethoxysilane, di-tert-butyldimethoxysilane, cyclohexylisopropyldimethoxysilane or dicyclopentyldimethoxysilane. The catalyst system has high activity and can desirably control the content of xylene solubles in the polymer to be in the range of 0.6 wt % to 3.0 wt %, thereby providing the polypropylene product with high degree of crystallinity. The external electron donor is preferably dicyclopentyldimethoxysilane. The embodiments disclosed in the patent demonstrate that, compared with several other external electron donors, dicyclopentyldimethoxysilane has the advantages of the lowest content of xylene solubles in the polymerization product and the highest isotacticity. In addition, Patent Nos. EP0419249, EP0565173, EP0657476, EP0844260, U.S. Pat. No. 5,166,340, U.S. Pat. No. 5,192,732 disclose external electron donors of organosiloxane types for catalytic olefin polymerization, all of which are incorporated herein by reference.
Although the external electron donors of organosiloxane type used for catalytic olefin polymerization have been reported in many published patents, and the typical external electron donors with cyclohexylmethyldimethoxysilane and diphenyldimethoxysilane as representative have been widely used in industrial production of polypropylene, the isotacticity of the polymers are generally in the range of 95% to 99%, for stereospecific polymerization of olefins, especially stereo specific polymerization of propylene, in the prior art. Further improvement of the isotacticity of the polymerization products still has great significance for production of some special products with high crystallinity and rigidity, so as to further improve mechanical properties of the polymer materials.